1. Field of the Invention
The invention relates to a kit for a knee joint prosthesis.
2. Description of the Prior Art
Knee joint prostheses are already obtainable on the market in a wide variety. The tendency is increasingly towards designing the prostheses to be modular in order to allow the surgeon the possibility where appropriate even during the operation of taking the actual anatomical conditions of the patient (e.g. quality of the bone, quality of the ligaments, etc.) into consideration in an ideal manner in the choice of the type of the prosthesis.
In knee joint prostheses which comprise a tibia part in which a meniscus part is provided on the surface facing the femur, a distinction in principle between the following four types should be made with respect to the kind of movability of the meniscus part relative to the surface of the tibia part facing the femur: prostheses with a meniscus part which is arranged to be immobile relative to this surface of the tibia part, prostheses with a meniscus part which is arranged to be exclusively rotatable relative to this surface of the tibia part, prostheses with a meniscus part which is arranged to be exclusively translationally displaceable relative to this surface of the tibia part, and prostheses with a meniscus part which is arranged to be both rotatable and translationally displaceable relative to this surface of the tibia part.
Whereas the individual types of such knee joint prosthesesxe2x80x94taken by themselvesxe2x80x94are already known, it is at least the case that those meniscus parts which are arranged to be immobile relative to the surface of the tibia part facing the femur are regularly connected firmly and unreleasably to or to be releasable only with great difficulty from the tibia part, e.g. with the help of snap connections, such as are shown in EP-A-0,923,916. A prosthesis in which the meniscus part is arranged to be exclusively translationally displaceable relative to the surface of the tibia part facing the femur is for example known from EP-A-0,913,132. A prosthesis in which the meniscus part is arranged to be both rotatable and translationally displaceable relative to the surface of the tibia part facing the femur is for example known from EP-A-0,519,873.
Disadvantageous is however that different tibia parts are required at least for those prostheses in which the meniscus part is firmly connected to the tibia part, e.g. by means of the snap connections, and for those prostheses in which the meniscus part is movablexe2x80x94in whatsoever wayxe2x80x94relative to the tibia part. The tibia part for the immobile meniscus part must in addition have additional means for the securing of the meniscus part (e.g. projections which enter into a snap connection with corresponding projections at the meniscus part), which makes the manufacture of the tibia part and of the meniscus part more complicated and expensive and also often leads to the tibia plateau of the tibia part on which the meniscus part lies becoming thinner. In addition the meniscus part is also formed to be thinner at its periphery, that is, where it comes to lie over the projections (for forming the snap connection) of the tibia part, which on the one hand is more complicated and expensive in the technical manufacture and on the other hand partly exposes the meniscus part to extreme stresses in these regions.
The object of the invention is therefore to propose a kit for a knee joint prosthesis which does not have the above-named disadvantages, but at the same time however maintains the intra-operative flexibility for the operating surgeon with respect to the type of knee joint prosthesis to be implanted.
The kit in accordance with the invention comprises a plurality of guiding elements which are formed in such a manner that the respective guiding element is in engagement with the tibia part and with the meniscus part when the prosthesis is assembled and determines the movability of the meniscus part relative to the tibia part. It is thusxe2x80x94for a given meniscus partxe2x80x94possible to determine through the choice of a corresponding guiding element whether the meniscus part is arranged immovably relative to the tibia part when the prosthesis is assembled or whether the meniscus part is arranged to be movable relative to the tibia part, and thus can be moved on the tibia plateau. In this the movability on the tibia plateau can mean that the meniscus part is either exclusively rotatable or exclusively translationally displaceable or both rotatable and translationally displaceable.
Moreover, only one type of tibia part is required for the different types of knee joint prostheses (at most different sizes, but no longer different types however), just because the movability is determined by the guiding element. Through this the cost and complexity in the manufacture is considerably reduced, since on the one hand different types of tibia parts need not be manufactured. Moreover, the technical manufacturing cost and complexity for projections, etc. for the securing of meniscus parts by means of snap connection or the like is also avoided. The same holds for the meniscus parts; thus no special meniscus parts need be manufactured for a knee joint prosthesis in which the meniscus part is to be immobile relative to the tibia part. The remaining disadvantages which have been arising up to now (lower thickness of the tibia plateau and/or partly extreme stressing in the peripheral regions of the meniscus part) also disappear.
At the same time however the intra-operative flexibility for the operating surgeon in regard to the type of the knee joint prosthesis to be implanted remains completely preserved. The surgeon can namelyxe2x80x94in accordance with the actual anatomical conditionsxe2x80x94still decide during the operation where necessary that a different type of knee joint prosthesis than the type which was planned prior to the operation represents the ideal care for the patient without it being necessary to provide additional prosthesis parts which are not contained in the kit.
In an advantageous exemplary embodiment of the kit in accordance with the invention the tibia part has a bore in its surface facing the femur part which is provided with a cut-out. The individual guiding elements have in each case a pin which protrudes into this bore in the tibia part when the prosthesis is assembled. In addition the guiding elements have in each case a guiding part which is in engagement with a corresponding guiding surface at the meniscus part when the prosthesis is assembled. Some of the pins have a projection which comes to lie in the cut-out of the bore, whereas other pins have no such projection.
Those guiding elements which have a pin with a projection are rotationally fixedly seated in the bore of the tibia part after the introduction of the pin there. This also holds for the guiding part belonging to the respective guiding element and thus also for the meniscus part which is in engagement with the guiding part. On the contrary those guiding elements which have no such projection are rotatably journalled, which also holds correspondingly for the guiding part and thus also for the meniscus part which is in engagement with the guiding part. The possibility of the translational displacement of the meniscus part along the guiding part will be discussed below.
In a further advantageous exemplary embodiment the meniscus part has an elongate hole in which the guiding part is received when the prosthesis is assembled. This exemplary embodiment is distinguished insofar as it can be particularly simply assembled during the operation. First, namely, the tibia part is usually secured to the prepared tibia and the femur part to the prepared femur. Thenxe2x80x94depending on the prosthesis typexe2x80x94a corresponding guiding element can be inserted into the bore of the tibia part. Once this has taken place, then the meniscus part can be pushed over the guiding part of the guiding element and finally the knee joint repositioned.
It is particularly advantageous when guiding elements with different lengths of the guiding part are provided in the kit, so that for one and the same meniscus part, depending on the length of the guiding part, a translational movement of the meniscus part is either possible or is not possible when the prosthesis is assembled. This means that for a given meniscus part (e.g. with predetermined length of the elongate hole) it can be determined by the choice of the guiding element alone whether a translational displacement of the meniscus part along the guiding part should be permitted or not. This decision often depends on the state of the ligament apparatus. If such a translational displacement is to be permitted, then the length of the guiding part should be chosen to be shorter than the length of the elongate hole. If on the contrary no translational displacement of the meniscus part along the guiding part is to be permitted, then the length of the guiding part is to be chosen to be equal to the length of the elongate hole.
In a further advantageous exemplary embodiment of the kit in accordance with the invention the pin of the guiding element is formed as a hollow pin for the reception of a pin of a stabilizing element. This stabilizing element protrudes between the condyles of the femur part when the prosthesis is assembled and has means for the varus-valgus stabilizing. These means for the varus-valgus stabilizing can e.g. be stabilizing surfaces which cooperate with corresponding counter-surfaces. On the other hand the guiding element with the hollow pin can also be used alone, so that a guiding element which is formed in this manner does optionally allow the reception of a stabilizing element of this kind, but otherwise functions exactly as a guiding element in which the pin is formed as a full pin.